This invention relates to synchronizing images in color electrophotographic printing machines.
Electrophotographic marking is a well-known, commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a charged photoreceptor with a light image representation of a desired document. The photoreceptor is discharged in response to that light image, creating an electrostatic latent image of the desired document on the photoreceptor""s surface. Toner particles are then deposited onto that latent image, forming a toner image, which is then transferred onto a substrate, such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a permanent record of the original representation. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of other images.
The foregoing broadly describes a black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. For example, in one color process, referred to herein as the REaD IOI process (Recharge, Expose, and Develop, Image On Image), a charged photoreceptive surface is exposed to a light image which represents a first color, say black. The resulting electrostatic latent image is then developed with black toner particles to produce a black toner image. The charge, expose, and develop process is repeated for a second color, say yellow, then for a third color, say magenta, and finally for a fourth color, say cyan. The various color toner particles are placed in superimposed registration such that a desired composite color image results. That composite color image is then transferred and fused onto a substrate.
The REaD IOI process can be performed in various ways. For example, in a single-pass printer wherein the composite image is produced in a single pass of the photoreceptor. This requires a charging, an exposing, and a developing station for each color of toner. Single-pass printers are advantageous in that they are relatively fast. However, they are also relatively expensive since multiple charging, exposing, and developing stations are required. An alternative to single-pass color printing is multiple-pass color printing. In a multipass color printer an image for one color component is produced in one pass, another color component is produced in the next pass, and so on. However, in both types of printing engine mentioned above, it is very important that the color components are properly registered. Registration defects produce final images which are distorted and, more importantly, incorporate serious quality defects.
One way of exposing the photoreceptor is to use a Raster Output Scanner (ROS). A ROS is typically comprised of a laser light source (or sources), a pre-polygon optical system, a rotating polygon having a plurality of mirrored facets, and a post-polygon optical system. In a simplified description of the ROS operation, a collimated light beam is reflected from facets of an optical polygon and passes through imaging elements that project it into a finely focused spot of light on the photoreceptor surface. As the polygon rotates, the focused spot traces a path on the photoreceptor surface referred to as a scan line. By moving the photoreceptor as the polygon rotates, the spot scans a raster of lines on the surface of the photoreceptor. By modulating the laser beam with image information a predetermined latent image is produced on the photoreceptor.
Referring now to FIG. 1, a typical prior art exposure station includes a laser diode 8 that emits a laser beam 10 that is modulated in response to drive signals from a controller 12 applied on a line 9. As emitted from the laser diode, the laser beam 10 is divergent. A lens 14 collimates that diverging beam and directs the collimated beam through a cylindrical lens 16 that has focusing power only in the sagittal direction. After passing through the cylindrical lens 16 the laser beam is incident on a polygon 20 that includes a plurality of mirrored facets 22. The polygon is rotated at a constant rotational velocity by a motor (not shown) in a direction 24. The mirrored facets deflect the laser beam as the polygon rotates, resulting in a sweeping laser beam. A post-scan optical system 26 focuses the laser beam 10 to form a spot of circular or elliptic cross sectional shape on a moving photoreceptor 28. The post-scan optical system 26 is typically an F-theta lens design intended to correct for scan line nonlinearity (see below). In FIG. 1, the direction of photoreceptor motion would be into (or out of) the view plane. By properly modulating the laser beam 10 as the focused spot sweeps across the photoreceptor, a desired latent image is produced. That latent image is comprised of multiple scan lines, each of which is comprised of a plurality of image elements referred to as pixels.
In a color printer all scan lines are ideally geometrically straight lines that start at the same relative position on the photoreceptor and that have evenly spaced, identically sized pixels. Furthermore, each color component of the composite color image is perfectly registered with all of the other color components. Unfortunately, obtaining such ideal scan lines is very difficult. One particular problem is having each of the color components start at the proper place. Consider a yellow color component on a photoreceptor. This color component is comprised of a large number of scan lines, say 600 scan lines per inch. When the next color component (say magenta) is to be exposed the polygon facet that writes the first scan line of the magenta color component might not be in position to start a scan line. A delay of up to 1 scan lines might be required simply to bring the polygon facet into position to start a scan line.
Determining when a facet is in position to start a scan line is the job of a start-of-scan (SOS) detector. FIG. 1 shows a simple start-of-scan detector 36 that produces start-of scan signals when a facet is properly located. The start-of-scan detector 36 incorporates a fiber-optic element 44 that guides light received at its input end 46, which is in the scanning plane of the raster output scanner, to a photosensitive element (not shown). In response to a received light pulse produced by the sweeping scan line, the start-of-scan detector produces the start-of-scan signal on a line 38. That signal enables the controller 12 to begin producing a scan line at the correct relative location across the photoreceptor.
Determining where on the photoreceptor a color component is to be imaged is the job of a page detector. While many types of page detectors exist, probably the most common is the belt hole sensor. FIG. 2 illustrates one type of belt hole sensor. As shown, a light source 50 illuminates a belt hole 52 that passes through a photoreceptor 28 that moves in a direction 112. Opposite the light source is a light sensor 54. When the belt hole 52 is located between the light source and the light sensor, the illuminating light is detected. The light sensor 54 then signals the controller 12 via a line 56 that the belt hole is at a known location. The controller then controls the modulation applied to the laser diode 8 such that a latent image is produced at a desired location on the photoreceptor 28.
In view of the above, a technique of avoiding the delays required to bring the polygon facet into position to write the first scan line of a color image component would be beneficial.
The principles of the present invention provide for color electrophotographic printers having latent image positions synchronized with start-of-scan signals. A printer according to the present invention includes a laser source that projects a laser beam onto a multifaceted rotating polygon that sweeps that beam in a scan line across a moving photoreceptor. A start-of-scan detector produces start-of-scan signals when the laser beam is in position to write a scan line. A belt sensor produces belt signals when indicia on the photoreceptor pass a reference position. A controller receives the belt signals and the start-of-scan and then adjusts the photoreceptor velocity such that the belt signals are an integral multiple of the start-of-scan signals. On printers with more than one imaging station, the rotational velocities of each ROS producing the scan lines on the photoreceptor are appropriately varied so that all color image components start on coincident scan line positions. To reduce the impact of drive roller eccentricities on belt signal to start-of-scan signal timing, the perimeters of photoreceptor roller elements are sub-multiples of the photoreceptor length.